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Re: [PATCH] Improve performance when starting nginx with a lot of locations

Maxim Dounin
September 29, 2023 11:40PM
Hello!

On Fri, Sep 22, 2023 at 03:58:41PM +0900, Yusuke Nojima wrote:

> # HG changeset patch
> # User Yusuke Nojima <nojima@ynojima.com>
> # Date 1679555707 -32400
> # Thu Mar 23 16:15:07 2023 +0900
> # Node ID 6aac98fb135e47ca9cf7ad7d780cf4a10e9aa55c
> # Parent 8771d35d55d0a2b1cefaab04401d6f837f5a05a2
> Improve performance when starting nginx with a lot of locations
>
> Our team has a configuration file with a very large number of
> locations, and we found that starting nginx with this file takes an
> unacceptable amount of time. After investigating the issue, we
> discovered that the root cause of the long startup time is the sorting
> of the location list.

Interesting.

Could you please provide some more details about the use case,
such as how locations are used, and what is the approximate number
of locations being used?

In my practice, it is extremely uncommon to use more than 1k-10k
prefix locations (and even these numbers are huge for normal
setups). Further, since each location contains configuration for
all modules, such configurations are expected to require a lot of
memory (currently about 5-10KB per empty location, so about
50-100MB per 10k locations, and 0.5-1G per 100k locations).
Accordingly, using other approaches such as map (assuming exact
match is actually needed) might be beneficial regardless of the
sorting costs.

Nevertheless, swapping the sorting algorithm to a faster one looks
like an obvious improvement.

>
> Currently, the sorting algorithm used in nginx is insertion sort,
> which requires O(n^2) time for n locations. We have modified the
> sorting algorithm to use merge sort instead, which has a time
> complexity of O(n log n).
>
> We have tested the modified code using micro-benchmarks and confirmed
> that the new algorithm improves nginx startup time significantly
> (shown below). We believe that this change would be valuable for other
> users who are experiencing similar issues.
>
>
> Table: nginx startup time in seconds
>
> n current patched
> 2000 0.033 0.018
> 4000 0.047 0.028
> 6000 0.062 0.038
> 8000 0.079 0.050
> 10000 0.091 0.065
> 12000 0.367 0.081
> 14000 0.683 0.086
> 16000 0.899 0.097
> 18000 1.145 0.110
> 20000 1.449 0.122
> 22000 1.650 0.137
> 24000 2.155 0.151
> 26000 3.096 0.155
> 28000 3.711 0.168
> 30000 3.539 0.184
> 32000 3.980 0.193
> 34000 4.543 0.208
> 36000 4.349 0.217
> 38000 5.021 0.229
> 40000 4.918 0.245
> 42000 4.835 0.256
> 44000 5.159 0.262
> 46000 5.802 0.331
> 48000 6.205 0.295
> 50000 5.701 0.308
> 52000 5.992 0.335
> 54000 6.561 0.323
> 56000 6.856 0.333
> 58000 6.515 0.347
> 60000 7.051 0.359
> 62000 6.956 0.377
> 64000 7.376 0.376
> 66000 7.506 0.404
> 68000 7.292 0.407
> 70000 7.422 0.461
> 72000 10.090 0.443
> 74000 18.505 0.463
> 76000 11.857 0.462
> 78000 9.752 0.470
> 80000 12.485 0.481
> 82000 11.027 0.498
> 84000 9.804 0.523
> 86000 8.482 0.515
> 88000 9.838 0.560
> 90000 12.341 0.546
> 92000 13.881 0.648
> 94000 8.309 0.635
> 96000 8.854 0.650
> 98000 12.871 0.674
> 100000 8.261 0.698

This probably can be reduced to something like 3-5 data points.

>
> diff -r 8771d35d55d0 -r 6aac98fb135e src/core/ngx_queue.c
> --- a/src/core/ngx_queue.c Fri Mar 10 07:43:50 2023 +0300
> +++ b/src/core/ngx_queue.c Thu Mar 23 16:15:07 2023 +0900
> @@ -45,36 +45,103 @@
> }
>
>
> -/* the stable insertion sort */
> +/* merge queue2 into queue1. queue2 becomes empty after merge. */
> +
> +static void
> +ngx_queue_merge(ngx_queue_t *queue1, ngx_queue_t *queue2,
> + ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *))
> +{
> + ngx_queue_t *p1, *p2;

Nitpicking: there are various style issues here and in other places.

> +
> + p1 = ngx_queue_head(queue1);
> + p2 = ngx_queue_head(queue2);
> +
> + while (p1 != ngx_queue_sentinel(queue1)
> + && p2 != ngx_queue_sentinel(queue2)) {
> +
> + if (cmp(p1, p2) > 0) {
> + ngx_queue_t *next, *prev;
> +
> + next = ngx_queue_next(p2);
> + ngx_queue_remove(p2);
> + prev = ngx_queue_prev(p1);
> + ngx_queue_insert_after(prev, p2);
> + p2 = next;

Nitpicking: there is no need to preserve "next" here, since p2 is
always the head of queue2 and, and the next element can be
obtained by ngx_queue_head(queue2).

Also, instead of obtaining "prev" and using
ngx_queue_insert_after() it would be easier to use
ngx_queue_insert_before(). It is not currently defined, but it is
trivial to define one: it is an alias to ngx_queue_insert_tail(),
much like ngx_queue_insert_after() is an alias to
ngx_queue_insert_head().

> + } else {
> + p1 = ngx_queue_next(p1);
> + }
> + }
> + if (p2 != ngx_queue_sentinel(queue2)) {
> + ngx_queue_add(queue1, queue2);
> + ngx_queue_init(queue2);
> + }
> +}
> +
> +
> +/* move all elements from src to dest. dest should be empty before call. */
> +
> +static void
> +ngx_queue_move(ngx_queue_t *dest, ngx_queue_t *src)
> +{
> + *dest = *src;
> + ngx_queue_init(src);
> +
> + if (dest->next == src) {
> + dest->next = dest;
> + } else {
> + dest->next->prev = dest;
> + }
> + if (dest->prev == src) {
> + dest->prev = dest;
> + } else {
> + dest->prev->next = dest;
> + }
> +}

This function looks strange to me. There is the ngx_queue_add()
macro, which probably should be used instead (if needed).

> +
> +
> +/* the stable merge sort */
>
> void
> ngx_queue_sort(ngx_queue_t *queue,
> ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *))
> {
> - ngx_queue_t *q, *prev, *next;
> + ngx_queue_t merged[64], *p, *last;
>
> - q = ngx_queue_head(queue);
> -
> - if (q == ngx_queue_last(queue)) {
> + if (ngx_queue_head(queue) == ngx_queue_last(queue)) {
> return;
> }
>
> - for (q = ngx_queue_next(q); q != ngx_queue_sentinel(queue); q = next) {
> + last = merged;
>
> - prev = ngx_queue_prev(q);
> - next = ngx_queue_next(q);
> + while (!ngx_queue_empty(queue)) {
> + /*
> + * Loop invariant:
> + * merged[i] must have exactly 0 or 2^i elements in sorted order.
> + * For each iteration, we take one element from the given queue and
> + * insert it into merged without violating the invariant condition.
> + */
>
> - ngx_queue_remove(q);
> + ngx_queue_t carry, *h;
> +
> + h = ngx_queue_head(queue);
> + ngx_queue_remove(h);
> + ngx_queue_init(&carry);
> + ngx_queue_insert_head(&carry, h);
>
> - do {
> - if (cmp(prev, q) <= 0) {
> - break;
> - }
> + for (p = merged; p != last && !ngx_queue_empty(p); p++) {
> + ngx_queue_merge(p, &carry, cmp);
> + ngx_queue_move(&carry, p);
> + }
> + if (p == last) {
> + ngx_queue_init(last);
> + last++;
> + }
> + ngx_queue_move(p, &carry);
> + }
>
> - prev = ngx_queue_prev(prev);
> -
> - } while (prev != ngx_queue_sentinel(queue));
> -
> - ngx_queue_insert_after(prev, q);
> + /* Merge all queues into one queue */
> + for (p = merged + 1; p != last; p++) {
> + ngx_queue_merge(p, p-1, cmp);
> }
> + ngx_queue_move(queue, last-1);
> }

While bottom-up merge sort implementation might be more efficient,
I find it disturbing to use fixed array of queues without any
checks if we are within the array bounds.

Rather, I would suggest recursive top-bottom merge sort implementation
instead, which is much simpler and uses stack as temporary storage
(so it'll naturally die if there will be a queue which requires
more space for sorting than we have).

Please take a look if it works for you:

diff --git a/src/core/ngx_queue.c b/src/core/ngx_queue.c
--- a/src/core/ngx_queue.c
+++ b/src/core/ngx_queue.c
@@ -9,6 +9,10 @@
#include <ngx_core.h>


+static void ngx_queue_merge(ngx_queue_t *queue, ngx_queue_t *tail,
+ ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *));
+
+
/*
* find the middle queue element if the queue has odd number of elements
* or the first element of the queue's second part otherwise
@@ -45,13 +49,13 @@ ngx_queue_middle(ngx_queue_t *queue)
}


-/* the stable insertion sort */
+/* the stable merge sort */

void
ngx_queue_sort(ngx_queue_t *queue,
ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *))
{
- ngx_queue_t *q, *prev, *next;
+ ngx_queue_t *q, tail;

q = ngx_queue_head(queue);

@@ -59,22 +63,44 @@ ngx_queue_sort(ngx_queue_t *queue,
return;
}

- for (q = ngx_queue_next(q); q != ngx_queue_sentinel(queue); q = next) {
+ q = ngx_queue_middle(queue);
+
+ ngx_queue_split(queue, q, &tail);
+
+ ngx_queue_sort(queue, cmp);
+ ngx_queue_sort(&tail, cmp);
+
+ ngx_queue_merge(queue, &tail, cmp);
+}

- prev = ngx_queue_prev(q);
- next = ngx_queue_next(q);

- ngx_queue_remove(q);
+static void
+ngx_queue_merge(ngx_queue_t *queue, ngx_queue_t *tail,
+ ngx_int_t (*cmp)(const ngx_queue_t *, const ngx_queue_t *))
+{
+ ngx_queue_t *q1, *q2;
+
+ q1 = ngx_queue_head(queue);
+ q2 = ngx_queue_head(tail);

- do {
- if (cmp(prev, q) <= 0) {
- break;
- }
+ for ( ;; ) {
+ if (q1 == ngx_queue_sentinel(queue)) {
+ ngx_queue_add(queue, tail);
+ break;
+ }
+
+ if (q2 == ngx_queue_sentinel(tail)) {
+ break;
+ }

- prev = ngx_queue_prev(prev);
+ if (cmp(q1, q2) <= 0) {
+ q1 = ngx_queue_next(q1);
+ continue;
+ }

- } while (prev != ngx_queue_sentinel(queue));
+ ngx_queue_remove(q2);
+ ngx_queue_insert_before(q1, q2);

- ngx_queue_insert_after(prev, q);
+ q2 = ngx_queue_head(tail);
}
}
diff --git a/src/core/ngx_queue.h b/src/core/ngx_queue.h
--- a/src/core/ngx_queue.h
+++ b/src/core/ngx_queue.h
@@ -47,6 +47,9 @@ struct ngx_queue_s {
(h)->prev = x


+#define ngx_queue_insert_before ngx_queue_insert_tail
+
+
#define ngx_queue_head(h) \
(h)->next



--
Maxim Dounin
http://mdounin.ru/
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Subject Author Views Posted

[PATCH] Improve performance when starting nginx with a lot of locations

Yusuke Nojima 368 September 22, 2023 03:00AM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Maxim Dounin 85 September 29, 2023 11:40PM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Yusuke Nojima 69 October 04, 2023 09:52PM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Илья Шипицин 88 October 05, 2023 02:18AM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Yusuke Nojima 77 October 05, 2023 10:40PM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Maxim Dounin 66 October 10, 2023 06:58PM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Yusuke Nojima 70 October 11, 2023 06:34AM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Sergey Kandaurov 66 October 17, 2023 07:26AM

Re: [PATCH] Improve performance when starting nginx with a lot of locations

Maxim Dounin 90 October 17, 2023 10:04PM



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